1. Field of the Invention
The present invention relates to a liquid crystal display device (or “LCD device”). Especially, the exemplary embodiment relates to a liquid crystal display device and a driving method thereof for reducing heat and electric consumption power of the data driving circuit.
2. Discussion of the Related Art
The liquid crystal display device presents video images by controlling the light transparent ratio of the liquid crystal cells according to the input video signals. The active matrix type liquid crystal display device controls the video data actively by switching the voltage of the video data supplied to the liquid crystal cells using thin film transistors (TFTs) disposed at each liquid crystal cells (Clc) as shown in FIG. 1. Therefore, the active matrix type can ensures a high display quality. In the FIG. 1, the reference character “Cst” denotes a storage capacitor for maintaining the charged data voltage to the liquid crystal cell (Clc). The reference numerical ‘D1’ denotes data lines supplying the data voltages and the reference numerical ‘G1’ denotes gate lines supplying scan voltages.
To decrease offset components of the direct current and to reduce the degrade of the liquid crystal material, the liquid crystal display device is generally driven by the inversion method in which the polarity of data voltage applied to the neighboring liquid cells is opposite respectively and the polarity of data voltage applied to the same liquid cell is alternated in each frame. When the polarity of data voltage is alternated, the swing width of data voltage supplied to the data lines is large and the data driving circuit requires large electric current so that the data driving circuit is overheated and the consumption electric power will be increased.
To reduce the swing width of data voltage supplied to the data lines, to prevent the data driving circuit from being overheated and to reduce consumption electric power, was suggested that the charge share circuit or the precharging circuit is applied to the data driving circuits. However, the effect of this technique is not sufficiently met to the user's requirements.
FIG. 2 is the waveform illustrating the control of the data voltage using the conventional charge share circuit.
Referring to FIG. 2, the period of pulse of source output enable (SOE) signal for controlling the output of the data driving circuit is the 1 horizontal period. The data driving circuit supplies the charge share voltage to the data lines during the high logic period of the source output enable (SOE) signal, that is, during the pulse width period. During the low logic period of the source output enable (SOE) signal, the data driving circuit supplies positive or negative data voltage to the data lines. The data driving supplies the charge share voltage to the data line with synchronizing to the pulse of source output enable (SOE) signal in the frequency of the 1-horizontal period or the 2-horizontal period according to the kind of drive IC (integrated circuit), regardless of the polarity of the data voltage. In FIG. 2, the gate shift clock (GSC) signal is the clock signal for controlling the shift operation. The polarity control signal (POL) is the control signal for controlling the polarity of the data voltage outputted form the data driving circuit.
The charge share control generates the electric current of the data driving circuit smaller than the case in which the data voltage is supplied from the positive polarity data voltage to the negative polarity data voltage or vice versa. However, as the swing width of the data voltage after and before the charge share voltage, the electric current of the data driving circuit is still high. Especially, when the polarity of data voltage is changed and the polarity of data voltage is changed from the black scale value to the white scale value, the electric current flowing in the data driving circuit is rapidly increased.
When the polarity of the data voltage is alternated by the inversion method, the absolute amount of the charging voltage to the liquid crystal cell for the positive polarity data voltage and the absolute amount of the charging voltage to the liquid crystal cell for the negative polarity data voltage are different. Therefore, the display quality can be inferior.
This is explained referring to the FIG. 3. Assume that firstly the liquid cell is charged with the positive polarity data voltage (+Vp), and then it is charged with the negative polarity data voltage (−Vp) for representing the same gray-scale of the positive polarity data voltage (+Vp). After charging the positive polarity data voltage, the liquid cell holds the voltage (Vp(+)) of which absolute value is lowered with ΔVp by the parasitic capacitance of the TFT. Whilst, after charging the negative polarity data voltage, the liquid crystal cell holds the voltage (Vp(−)) of which absolute value is increased with ΔVp by the parasitic capacitance of the TFT. Therefore, the liquid cell of the normally black mode LCD device transmits the light with higher light transparent ratio when the negative polarity data voltage is charged than when the positive polarity data voltage is charged. In the normally black mode, the light transparent ration of the liquid crystal cell is increased as the voltage charged at the liquid crystal cell is higher. In the interim, the liquid crystal cell of the normal white mode LCD device transmits the light with lower light transparent ratio when the negative polarity data voltage is charged than when the positive polarity data voltage is charged. In the normally white mode, the light transparent ration of the liquid crystal cell is decreased as the voltage charged at the liquid crystal cell is lower.
The display quality of a liquid crystal display device may be degraded at certain data pattern according to the relationship between the polarity pattern of data voltage charged to the liquid crystal cells and the gray-scale value of the data. Hereinafter, this data pattern of degrade in a liquid crystal display device is defined as the “weakness pattern”. The representative causes of the quality degrade are the greenish phenomenon on display screen and the flicker in which the luminescent of display panel is periodically changed.
The FIGS. 4 and 5 illustrate the representative examples for the weakness pattern of the greenish shown in the display screen.
Referring to FIG. 4, one example for the weakness pattern of the greenish is the data pattern in which the gray scale of the data supplied to the pixels of the odd columns are white and the gray scale of the data supplied to the pixels of the even columns are black. When this kind weakness pattern is inputted, further if the LCD device is driven in the Vertical 2-dot and Horizontal 1-dot inversion method (V2H1), the liquid crystal display device may have greenish pattern more easily. In the Vertical 2-dot and Horizontal 1-dot inversion method (V2H1), the polarity of the data voltages charged to the liquid crystal cells is inverted (altered) at every two vertical dots (or 2 liquid crystal cells) of the display panel and at every one horizontal dot (or 1 liquid crystal cell) in one frame period.
In FIG. 4, as all data voltages of green (G) data which mainly makes an effect to the luminescence among the red (R), green (G) and blue (B) data at 1st, 2nd, 5th, and 6th lines (L1, L2, L5 and L6) are negative polarity data voltages, a greenish is shown at the lines. This greenish phenomenon is caused because the polarity of the green data has only one type (negative or positive) of polarity.
Referring to FIG. 5, for another example of the greenish weak pattern, the gray scale of the data supplied to the sub-pixels of the odd numbered columns is the white, and the gray scale of the data supplied to the sub-pixels of the even numbered columns is black. When this kind weakness pattern is inputted, further if the LCD device is driven in the Vertical 2-dot and Horizontal 1-dot inversion method (V2H1), the liquid crystal display device may have greenish pattern more easily.
FIG. 6 illustrates an example for occurring the weak pattern of flicker.
Referring to the FIG. 6, the example for the weak pattern of flicker is the mosaic pattern in sub-pixel unit in which the gray scale of data voltage of the every each other 1 pixel in horizontal and vertical directions is alternated with white and black gray scales. When this type of weakness pattern is inputted, further if the LCD is driven in the Vertical 1-dot and Horizontal 1-dot inversion method (V1H1), the display screen of this LCD panel can have flicker phenomenon easily. For the Vertical 1-dot and Horizontal 1-dot inversion method (V1H1), the data voltage of every neighboring liquid crystal cells in horizontal and vertical directions have reversed polarity respectively. In this case, all data voltages of the white gray scale in one frame period are positive data voltages and all data voltages of the white gray scale in the next farm period are also positive data voltages. Therefore, the luminescence of display screen can be changed in each frame period.
In addition, if the polarity of data voltage supplied to the liquid cell of the LCD device is kept in any one polarity for a long time, when the screen is changed, an incidental image in which the former image is shown incidentally can be appeared on the screen easily. As the incidental image phenomenon charges the same polarity voltage to the liquid cell continually, it is also defined as the “DC image sticking”. One example of the incidental image is occurred when the interlace type data voltage is supplied to the LCD device. The interlace type data voltage (or interlace data) includes only the odd line data voltage supplied to the liquid cells in the odd lines for the odd frame period. Further, the interlace data includes only even line data voltage supplied to the liquid cells in the even lines for the even frame period.
FIG. 7 illustrates an example of the interlace data. Assume that the liquid cell supplied with the data voltage shown in FIG. 7 is any one of the liquid cell disposed in an odd line.
Referring to FIG. 7, for the odd frame period, the positive voltages are supplied to the liquid cells and the negative voltages are supplied to the liquid cells for the even frame period. In the interlace mode, the high positive data voltages are supplied to the liquid cells disposed in the odd lines only for the odd frame period. For 4 frame periods, the positive data voltages are superior to the negative data voltages as the waveform shown in the box, so that the DC image sticking is occurred.
FIG. 8 is the image illustrating the experience result of the DC image sticking occurred by the interlace data. If the original image as the left image shown in the FIG. 8 is supplied to the LCD panel for predetermined time period, the data voltage having the same polarity is repeatedly charge to the liquid cells. As a result, after the original image data as the left image of FIG. 8, a intermediated gray scale, for example 127 gray scale data voltage is supplied to all liquid cells of the LCD panel, the original image pattern will not be represented clearly, that is there is the DC image sticking is occurred.
For another example of the DC image sticking, when the same image is moved or scrolled with a constant speed, according to the relationship between the size of the scrolled (moved) picture and the scroll speed (moving speed) the same polarity voltage is repeatedly charge to the same liquid cells. As a result, there is a DC image sticking phenomenon. This example is as shown in FIG. 9. FIG. 9 is the image illustrating the experience result of the DC image sticking when a deviant crease line pattern and a text pattern are moving with constant speed.